1. Technical Field
The present invention relates to printing systems and, in particular, to a system and method for initiating Start-of-Plot in a printing system such that an image is printed in accurate registration with previously printed images on a moving substrate.
2. Discussion of the Prior Art
Systems for controlling the relative positions of a moving substrate and an element or elements which operate on the moving substrate have been in use for some time. Control systems of this type typically use marks or indicia which are printed on the substrate material. These marks are scanned by a sensor as the substrate is fed past the operating element. When a mark passes the sensor in such a manner as to produce a signal indicative of an error in position between the mark and the operating element, the sensor, in conjunction with control circuitry, generates a correction signal and an adjustment is made.
U.S. Pat. No. 4,569,584, issued Feb. 11, 1986 to St. John et al., discloses a color electrographic recording device which produces a composite color image on a recording medium. The St. John printer transports the recording medium along a predetermined path. A print station in the transport path of the medium includes a recording head with an electrode which forms an electrostatic latent image on the medium. A number of developing stations in the transport path develop the latent image into a corresponding visible component image of a respective color.
The registration system disclosed in the St. John patent utilizes a series of solid, spaced-apart tracking marks which are printed on the print medium adjacent both of its edges. The tracking marks are printed such that a known, constant number of optical encoder pulses will be generated between adjacent marks. The tracking marks are observed electro-optically as the print medium moves through the device. The photosensors, in conjunction with appropriate electronics, are used to determine whether the number of pulses generated by the optical encoder, relating to the spacing between adjacent tracking marks, is the same or different from the expected constant value. Any differences between the observed value and the expected constant value are processed to form an error sample representative of the differences. A number of these error samples are then averaged to produce an error correction signal which corresponds to an average of the physical longitudinal shrinkage or expansion of the print medium that has occurred between the time the device printed the tracking marks and the later time that the marks are observed. The error correction signal is utilized to prevent image misalignment.
The St. John patent also teaches that the series of tracking marks used to maintain print image registration may be preceded by a series of initializing marks which is used to determine the point on the moving substrate at which printing of an image, or Start-of-Plot (SOP), will commence. This determination is made from the transition from the last initializing mark to the first tracking mark. When this transition is detected, the SOP point on the substrate can be determined. For example, as disclosed in the St. John patent, the initializing marks may be one-third the width of the tracking marks. For a photosensor detection cycle between the last narrower initializing mark and the first wider tracking mark, less encoder pulses will be detected than are detected for the normal cycle between adjacent tracking marks. This difference in the number of detected pulses is utilized to determine where on the substrate Start-of-Plot will begin.
The St. John patent also discloses a pair of solid, continuous tracking lines, one of which is formed near each edge of the print medium. These solid, continuous lines, in conjunction with appropriate photosensors and servo circuitry, are used for corrective lateral translation of the print medium.